What gems, what stats etc?

Hey, I'm lvl 67 atm, but I am going to be the MT of our guild when we start recruiting.
I am wondering what gems are good.
Like lets say I need more hp, then I would buy the +12 sta gems obviously, but what if I need def rating? Dodge or whatever, what are the best and 2nd best gems (if I cant afford the best )?
Also, how much defense should I get to be crit immune?
And what are good dodge and block etc.. %? (I mean whats a normal percentage to bloc, dodge etc.. for a T6 geared and like T4 geared pala)
Also, whats a the min. spell damage value for tanking karazan, and whats the min. value for tanking the best instances, like BT and hyjal?

Just so that I know what I should get more.
Thanks in advice!
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I am paladin.

I would recommend reading the stickies in this forum as well as the gear forum, and then if you have any more specific questions asking again. Thanks!
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The danger must be growing, for the rowers keep on rowing!